Japanese Patent Laid-Open No. 2000-341143 discloses an example of a structure of a conventional amplifier that reduces a dependency of the characteristic on a load impedance. The amplifier disclosed in Japanese Patent Laid-Open No. 2000-341143 conducts detection at plural points on a transmission line between a power amplifier and an antenna, detects the occurrence of a standing wave, that is, a mismatching of a load on the basis of a difference between the detected levels, and limits an input of the power amplifier.
Also, U.S. Pat. No. 6,720,831 discloses a structure that detects a voltage outputted from a final-stage transistor, and feeds back the detected voltage to a bias voltage of the final-stage transistor.